The long-dormant Chaiten volcano blasted ash some 20 miles (30 kilometers) into the Andean sky on Tuesday, forcing thousands to evacuate and fouling a huge stretch of the South American continent.

The thick column of ash climbed into the stratosphere and blew eastward for hundreds of miles (kilometers) over Patagonia to the Atlantic Ocean, forcing schools and a regional airport to close. Citizens of both countries were advised to wear masks to avoid breathing the dangerous fallout.

The five-day-old eruption is the first in at least 9,000 years for the volcano in southern Chile, according to volcanologists at the Smithsonian Institution in Washington.

Chilean officials ordered the total evacuation of Chaiten, a small provincial capital in an area of lakes and glacier-carved fjords just six miles (10 kilometers) from the roiling cloud.

Also emptied was the soot-coated border town of Futaleufu, about 75 miles (120 kilometers) from the volcano.

The gritty, gray-white blizzard covered houses, roads and even cattle. People wrapped cloths around their faces and wore surgical masks as they slogged through the mess.

About a 1/2 inch (1 centimeter) of ash coated the Argentine tourist town of Esquel, a Patagonian resort favored by backpackers and skiers at the foot of the Andes whose airport and schools have been closed since Saturday.

The fallout covered a third of Argentina's Minnesota-sized province of Chubut, provincial Gov. Mario Das Neves said.

While volcanologists around the world eagerly awaited data on the scope of the eruption, one local expert got an up-close look when he accompanied police and air force teams over the 3,950-foot (1,200 meter) mountain.

Volcanologist Juan Cayupi told The Associated Press by telephone that Chaiten's two small craters have morphed into a large, single crater, and "a large amount of ash, particles, gas" was pouring out.

Lava was rising within this crater but has not yet spilled over, said Luis Lara, another volcanologist with the government's Geology and Mining Service.

The few remaining residents of Chaiten were transferred to two navy ships Tuesday, a day after President Michelle Bachelet visited the town, pledging financial help for people whose homes were damaged or livestock died after foraging on ash-covered grass.

Experts said it is too early to say whether the volcano will affect the world's climate.

So far, Chaiten has emitted only a few thousand tons of sulfur dioxide, "which is very small," said Simon Carn, a University of Maryland-Baltimore Campus volcanologist who uses satellites to measure volcanic gases.

In general, a volcano must spew at least 1 million tons of sulfur dioxide into the stratosphere to have a global effect on climate, said Alan Robock, a Rutgers University professor who co-authored a book on the subject.

After eruptions of unusual size, sulfur dioxide, converted into sulfuric acid, can form a thin white cloud in the atmosphere that reflects sunlight away from Earth.

The Philippines' Mount Pinatubo produced a brief cooling of the climate after spewing 20 million tons of sulfur dioxide in 1991.

But Robock said this volcano is so close to the South Pole that any cooling would likely be limited to the Southern Hemisphere.

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Associated Press Science Writer Seth Borenstein in Washington, D.C., contributed to this report.